This invention relates to an electrothermal transfer sheet, and more particularly to a thermal transfer sheet for use with an electrothermal transfer printing method.
An electrothermal transfer printing method is a method in which printing is carried out by utilizing heat which is generated when an electric current is applied by an electrode head. With this printing method, an electrothermal transfer sheet comprising a substrate sheet, a resistor layer formed on one surface of the substrate sheet, capable of generating heat when an electric current is applied thereto by an electrode head, and a dye layer formed on the other surface of the substrate sheet, comprising a dye, such as a sublimable dye, transferable to an image-receiving sheet upon application of heat; and an electrothermal transfer sheet whose substrate sheet itself has electroconductivity and can serve as a resistor layer have been conventionally used. In particular, the latter transfer sheet has improved thermal sensitivity.
A film of a thermoplastic resin such as polyethylene-terephthalate is used as the substrate sheet and/or the resistor layer of the above-described conventional electrothermal transfer sheets. To conduct electrothermal transfer printing, an electrode head is used, as a heat-application means, to apply an electric current to the transfer sheet so as to directly generate heat in its resistor layer. Although thermal energy can thus be effectively utilized when printing is carried out by this printing method, the generated heat tends to partially accumulate in the electrothermal transfer sheet. The electrothermal transfer printing method brings about such partial accumulation of heat much easier than the printing method which employs a thermal head as a heat-application means. Since the thermoplastic resins which are used for the substrate sheet and/or resistor layer of the conventional electrothermal transfer sheets have low heat resistance, the conventional transfer sheets cannot fully endure the practical electrothermal transfer printing.
In other words, the conventional electrothermal transfer sheets cannot exhibit sufficient mechanical strength when heated, and suffer from problems of crumpling and breaking when printing is carried out. Moreover, the resistor layer and the substrate sheet are fused by the partially accumulated heat, and the fused material sticks to the surface of an electrode head to cause a short circuit. As a result, the electrode head partially generates an excessively high temperature of heat. Because of this heat generated, the resistor layer fuses and sticks to the electrode head, causing various problems; for instance, the electrode head cannot run smoothly when printing is carried out, and an image cannot be normally obtained.